1. Field of the Invention
The present invention relates to a sheet feeding device and an image forming apparatus equipped with the same, and in particular, to a construction for controlling an upper surface position of sheets contained in a sheet containing means.
2. Related Background Art
In recent years, image forming apparatuses, such as printers, copying machines, and facsimile apparatuses, are equipped with a sheet feeding device for feeding sheets one by one to an image reading portion or an image forming portion. As an example of this sheet feeding device, one adopting a retard separation system capable of separating sheets with high reliability over a range from low speed to high speed is in general use.
The retard separation system is a system in which sheets sent out from a pick-up roller serving as the sheet feeding means are fed one by one with a feed roller rotating in a sheet feeding direction and a retard roller in contact with the feed roller under a predetermined pressure and rotating in a direction opposite to the sheet feeding direction, i.e., so as to return sheets.
Apart from this, a conventional sheet feeding device adopts an ascent/descent system in which the pick-up roller is capable of ascending and descending, and in which, when sheet feeding is to be performed, the pick-up roller is lowered to be brought into contact with the sheets to be sent out, whereas when the operation of sending out sheets is not to be performed, the pick-up roller is raised so as to be spaced apart from the sheets. By thus spacing the pick-up roller apart from the sheets, the pick-up roller constitutes no load during transport and separation of the sheets; further, the sheets returned by the retard roller can be reliably returned to the cassette, thereby achieving an improvement in separability.
FIG. 8 shows a construction of a conventional sheet feeding device adopting this pick-up roller ascent/descent system; in this device, sheets supported by an inner plate 41A incorporated in a sheet feeding cassette 4A are sent out by a pick-up roller 56. This pick-up roller 56 is rotatably supported by a rotation end portion of a roller holder 58 rotatably mounted to a shaft 57a of a feed roller 57, and is capable of ascending and descending.
The roller holder 58 is equipped with a sensor flag 59, which is detected by an optical sensor 60. A position where the optical sensor 60 detects the sensor flag 59 is the position where a proper sheet feeding pressure is applied to an upper surface of the sheets P by the pick-up roller 56 at the time of feeding.
The inner plate 41A is capable of ascending and descending while supporting the sheets; when the sheets P have been reduced as a result of feeding and the optical sensor 60 can not detect the sensor flag 59, the inner plate 41 ascends; and when the optical sensor 60 detects the sensor flag 59, the ascent of the inner plate 41A is stopped based on this detection. As a result, the sheets P contained in the sheet feeding cassette 4A are sent out; each time the height of the uppermost surface of the sheets becomes lower than a given height, the inner plate 41A is raised, making it possible to maintain the uppermost surface of the sheets P substantially at a predetermined level at which the proper sheet feeding pressure is applied to the sheets P.
Incidentally, in such a conventional sheet feeding device, after sending out sheets P, the pick-up roller 56 ascends to be spaced apart from the sheets P1 as described above; this also applies to the case in which the last sheet in the sheet feeding cassette 4A has been sent out, so that, when the last sheet has been sent out, the pick-up roller moves to its initial position above the sheet feeding cassette.
Then, also, when sheets are supplied and the sheet supplying cassette 4A is attached to the apparatus main body, the pick-up roller 56 is situated at the initial position. When the pick-up roller 56 is thus situated at the initial position, the sensor flag 59 shields the optical sensor 60.
However, when the optical sensor 60 has already been thus shielded when the sheet feeding cassette 4A is attached, the control portion determines that sheet feeding is possible even if the uppermost surface of the sheets P has not yet reached the substantially predetermined level at which the proper sheet feeding pressure is applied to the sheets P. That is, when the optical sensor 60 is shielded, it is impossible to accurately judge the position of the sheets P.
Then, when it is thus impossible to accurately judge the position of the sheets P, sheet feeding operation is started although the sheets P have not reached the position where sheet feeding is possible. As a result, there is a fear of defective sheet feeding, or, in some cases, an excessive rise of the inner plate due to control thereof, which causes a problem such as deformation of components.